1. Field of the Invention
This invention relates to a zinc alkaline secondary battery such as the nickel-zinc secondary battery or silver-zinc secondary battery which operates with an alkaline electrolyte and uses zinc as a negative active material, and more particularly to a zinc alkaline secondary battery having an outer layer of cadmium metal and/or a cadmium compound formed on the outer side of the layer of the negative active material.
2. Description of the Prior Art
In recent years, remarkable dissemination of portable electronic and electric devices has come to accelerate rapid replacement of primary dry cells by secondary batteries, especially nickel-cadmium secondary batteries. As a more capable version of such secondary batteries, the zinc alkaline secondary battery has been drawing keen attention. This secondary battery enjoys the following advantages.
(1) Zinc, as a negative active material, possesses a high energy density per weight and is available at a low cost. PA0 (2) The operating voltage of this secondary battery is high and stable. PA0 (3) The secondary battery exhibits good low-temperature properties. PA0 (4) It operates safely without causing any environmental pollution.
This zinc alkaline secondary battery nevertheless has a disadvantage that its service life, namely the charge-discharge cycle life, is short.
It is held that this disadvantage is ascribable to the formation of dendritic zinc or readily shedding mossy zinc on the surface of the negative electrode. With a view to eliminating this disadvantage, improvements have been made in electrolytes, current collectors, active material compositions, separators, structures of negative electrodes, etc.
Inventions relating to improvements on the structures of negative electrodes, among others, are subsequently described.
For example, U.S. Pat. No. 4,022,953 discloses a negative electrode wherein the current collector thereof is coated with a first layer of cadmium metal and/or a cadmium compound and this first layer is further coated with a second layer formed of a zinc active material and cadmium metal and/or a cadmium compound.
Japanese Patent Publication No. 6690/1979 discloses a negative electrode wherein the central layer of a zinc active material has disposed on the outer side thereof a mixed layer formed of the hydroxide of an alkaline earth metal, particularly calcium hydroxide.
In the circumstance, the inventors have perfected a zinc alkaline secondary battery of a construction which is capable of inhibiting the zinc in the negative electrode from being dissolved out in the form of zincate ions, [Zn(OH).sub.4 ].sup.2-, into the electrolyte during the discharge of the secondary battery to notably elongate the cycle life of the secondary battery. To be specific, the zinc alkaline secondary battery of this invention is characterized by the fact that the negative electrode thereof uses as its base a layer of a zinc active material incorporating therein cadmium metal and/or a cadmium compound and has formed on the surface thereof with an outer layer formed of cadmium metal and/or a cadmium compound. This invention, accordingly, is believed to provide a novel zinc alkaline secondary battery which differs from any of the various versions of prior art mentioned above in terms of object and construction.